The rules, in this case are pretty clear. The spell never mentions mirrors or reflections, it “summons illusory duplicates." Spell names aren’t indicative of their mechanical effect. See Chill Touch.
There’s also vampire wizard statblock that has Mirror Image on its spell list.
It would be funny if the spell just failed though.
But do their mirror images show in a mirror, as the mirror image itself isn't a vampire?
What edition are we in? Because "I roll to disbelieve!" tells me that an illusion is all in the mind... >.>
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